I've got double shackles on my Chevy springs simply because it made mounting them easier. They've been there for 6 years or more and have NEVER caused any strange handling problems on or off road.

That said, they don't really do any good either <img src="/forums/images/graemlins/pfft.gif" alt="" />
(tiny bit of extra droop I guess, but nothing that would help you make an obstacle you wouldn't normally be able to do).

So from pretty close to personal experience, I'd say those are just a waste. Your money is much better spent on quality springs and/or lockers. <img src="/forums/images/graemlins/cheers.gif" alt="" />